1. Field of the Invention
The present invention relates generally to finishing machines and more particularly to vibratory finishing machines of the continuous feed type having discharge systems for controlling the retention time of workpieces within the tubs of the machines.
2. Prior Art
Vibratory finishing machines for smoothing and finishing workpiece surfaces by such operations as deburring, burnishing, descaling, and cleaning are well known, as is apparent from the disclosures in the referenced Finishing Patent, Orbital Patent, Dual Shaft Patent, and Continuous Feed Patent. Such machines commonly employ a movably mounted processing tub for receiving a quantity of workpieces and abrasive finishing media. A finishing action is imparted to the workpieces by vibrating the tub.
The type of abrasive media used in finishing operations varies substantially depending on the type of operation to be conducted. In many finishing operations, the abrasive finishing media comprises a multiplicity of modestly sized, generally triangular abrasive stones, and a small quantity of finishing liquid.
As is explained more fully in the above-cited patents, to which reference is made for a more complete description of finishing machines and techniques, the abrasive finishing media is conveniently separated from the workpieces after the media and the workpieces are discharged from the tub. After separation, the abrasive media is returned to the tub for reuse. The finishing liquid is usually drained off during separation of the media and workpieces and may also be returned to the tub for reuse. The finished workpieces are then normally conveyed to a separate unit or units for washing, drying, inspection and packaging.
Vibratory finishing machines are often categorized as being either of the batch type or the continuous feed type. In a batch-type operation, the machine is fully loaded, a finishing operation is carried out, and the machine is completely emptied. In continuous feed operation, media and workpieces are introduced into the tub near one end and are subjected to a finishing operation during orbital movement toward a discharge outlet at the opposite tub end. Continuous feed machines may be truly continuously fed, as by the use of charging conveyors or the like, or may periodically be fed with charges of media and workpieces. Regardless of how the machines are fed, they may continuously or periodically discharge quantities of the media-workpiece mixture through the tub outlet.
One problem with many prior continuous feed machine proposals is that the time during which workpieces are retained in the tub is not readily controllable. Workpieces of relatively soft metal or workpieces needing only minimal deburring require only short retention times to complete a finishing operation. Workpieces of harder metal or workpieces having many large burrs to be removed require longer retention times for satisfactory finishing. Where the required retention time of a particular operation differs from the design characteristics of the continuous feed machine in which the operation is to be performed, it may be necessary to operate the machine on a batch basis to effect adequate control of tub time.
The machines described in the referenced Continuous Feed Patent and Door Assembly Patent are provided with discharge systems for controlling retention time. These machines may be operated on a batch basis, if necessary, but are designed primarily for continuous operation wherein their discharge closures are opened and closed by a time control device preset to selected intervals of operation. Once a closure operating cycle has been decided upon, the operator adjusts the timing mechanism to close the discharge, typically for about 10 seconds, for processing workpieces in the tub. This is normally followed by a release period, typically about five seconds, for discharging part of the materials from the tub. This cycle of closure operation is continuously repeated and establishes an effective rate of material discharge from the tub which, in turn, determines the time span during which workpieces are retained in the finishing tub. In the event the finishing operation calls for longer or shorter processing times, the operator resets the time mechanism to provide correspondingly longer or shorter workpiece retention periods.
Although the closure control system described in the Door Assembly Patent operates quite satisfactorily to control workpiece retention time, the system is rather expensive. The control system required to vary the operating cycle and the associated safety systems (see the Object Sensor Patent) needed to prevent the crushing of workpieces trapped between the closure and tub outlet are relatively complex and add significantly to the cost of the machine.